Thermicity of the Decomposition of Oxygen Functional Groups on Cellulose-Derived Chars

The evolution of oxygen functional groups (OFGs) and the associated thermic effects upon heat treatment up to 800 °C were investigated experimentally as well as by theoretical calculations. A synthetic carbon with a carbonaceous structure close to that of natural chars, yet mineral-free, was derived from cellulose and oxidized by HNO3 vapor at different temperatures and for varied durations in order to generate char samples with different concentrations and distributions of OFGs. The functionalized samples were subjected to calorimetric temperature-programmed desorption measurements in correlation with an extensive effluent gas analysis, thereby focusing on the specific heat effects of individual OFG evolution. Interpretation of the experimental results was aided by density functional theory (DFT) calculations which allowed one to infer the thermal stability of different OFGs and the reaction energy associated with their evolution upon heating. Results showed that, with increasing temperature, H2O was released due to the loss of physisorbed water, the decomposition of clusters bound to carboxylic acids, and condensation reactions. The associated heat uptake amounted to about 100 kJ mol–1. Contrarily, the release of CO2, attributed to the decomposition of condensed acids, carboxylic acids, anhydrides, and lactones, resulted in a heat release of about 40 kJ mol–1. The most strongly pronounced thermic effects were detected for the release of CO, comprising highly exothermic effects due to the decomposition of condensed acids and carbonyls/quinones as well as endothermic effects attributed to anhydrides and phenols/ethers. Notably, anhydrides can be formed during the oxidative treatment as well as during heating by condensation of adjacent carboxylic acids. In the latter case, the two-step decomposition is overall highly exothermic, indicating the associated occurrence of pronounced carbon matrix rearrangements. DFT investigations suggest that these rearrangements not only affect the immediate OFG proximity but also involve several carbon sheets.

S2 S1 Supplementary Information on Char Oxidation by Molecular Oxygen

S1.1 Functionalization by Molecular Oxygen
In addition to the samples functionalization using HNO3 vapor, char oxidation by molecular oxygen was also performed. MH800 was placed on a frit in a vertical quartz reactor (inner   (1)). During the measurements, a base pressure of about 1×10 −9 mbar was maintained in the measurement chamber and a flood gun was used to compensate charging effects. For both energy regions, the spectra were recorded in fixed transmission mode with a pass energy of 200 eV and measurements were performed five times in succession with six to ten iterations. Finally, S5 calibration on the graphitic carbon peak at 284.5 eV and normalization to its maximum intensity were performed.

S2.3 TPD Measurements with an Extensive Evolved Gas Analysis
TPD measurements with an extensive evolved gas analysis (EGA) were carried out in a two-part quartz reactor (inner diameter 7mm/3mm, length 370mm + 330mm) ensuring a defined sample position. In each experiment 60 mg of char sample were placed between two quartz wool plugs.
The samples were heated with 5 °Cmin −1 to 800 °C in a constant flow of 50 mL min −1 N2 (99.999% purity). The sample temperature was detected by a type N thermocouple (± 1 °C) and downstream, the effluent gases were analyzed by a multi-channel analyzer (NGA 2000 MLT4 by Rosemount) which enabled the quantification of H2O, CO2, and CO in a range of 0% to 1%.

S2.4 Calorimetric TPD Measurements
Calorimetric TPD measurements were carried out analogous to the TPD-EGA measurements. In

S2.5 Deconvolution of Effluent Gas and Heat Flow Curves
The H2O, CO2, and CO release curves throughout TPD-EGA were deconvoluted assuming Gaussian distributions based on previous literature. 39 The assignment of EGA peaks to different OFGs is discussed in Section 3.
with a general offset 0 as well as area , full width at half maximum , and centre temperature as Gaussian parameters. However, as anhydrides are known to decompose into CO and CO2 of equal amounts, the deconvolution was further restricted by specifying equal areas for condensed

S10
Although the signal intensity of the COx TPD-EGA and heat flow curves of the less intensely oxidized chars (Fig. S3) was not sufficient to perform the deconvolution procedure, it is possible to check for consistency based on the OFG assignment (Table 2) and heat flow trends (Table 5) derived for the more extensively functionalized samples:  Table S2 for the different evolving gases. S13 Table S2. Assignment of fit contributions in the TPD curves (Fig. S4) Table S4 for the different OFG decompositions, and fitted curves (Fit) of qualities 2 determined according to Eq. (4). S16 Table S4. Assignment of contributions in the heat flow curves (Fig. S5)  employing the ridft module. The TPSS functional 7 was chosen in combination with the def2-SVP basis set. 8 The corresponding auxiliary basis set was taken from Eichkorn et al. 9 Furthermore, Grimme's D3 dispersion correction 10 was applied. The construction of the models was performed mostly with Python code using the CGAL (4.2) 11 library for the construction of the hypersurfaces, the pydlpoly code 12 as a molecular mechanics backend for preoptimization after prepositioning of the carbon atoms. In the MD simulations, the char models were equilibrated using ReaxFF as implemented in the ADF software suite (2018.103). 13,14 For the production level calculations, the extended tight binding methods (xTB, GFN2 parameterization) 15 were used, since due to the ReaxFF parameterization the model was even stable at high temperatures and did not decompose.
The inertness of the system treated on the ReaxFF level is due to the parameterization and can only be partially overcome by longer simulation times. S18 Figure S7. Selected char model used in the MD simulation containing mainly anhydrides as OFGs.